Some update on my test with ultra-productive gobs litters on a 250 years smaller worldgen-ed region (to avoid having too many civs, so gob attention wouldn't go too much to other things), embarking close to a dark fortress with 10000 gobs :
8: Nakootak, "Tormentconfined", dark fortress
Owner: The Larval Nightmares, goblins
Parent Civ: The Vice of Trials, goblins
lord: Ases Juggledwrung, human
296 humans
17 dwarves
13 elves
1 pale taupe devil
10128 goblins
5100 trolls
76 human prisoners
16 elf prisoners
And there was no more elf civ, so it was down to dwarves, humans and gobs only.
Looked like a nice dark fortress that would be completely unplayable if you visit it in adventure mode, but maybe good if sieges only happens when gobs pop skyrocket like that.
After 3 ingame years of building up my fortress and getting more than 100k wealth, i finally got a goblin siege, they sure took their time, but at least they came, that's some progress in comparison to my last forts in which no gobs ever tried
My military was ready, 20 dwarves (in 2 squads) that had by then some good sword, warhammer and marksdwarves skills from 3 years of training.
Let's see what the mighty evil legion of gobs throw at me ... 10 gobs and some animal guy that probably took the wrong direction when trying to get home
Oh well, so much for expectations, at least it beat my previous "biggest DF2014 siege ever" that had 8 gobs.
I guess then that dark fortress pop or your own pop does not really matter, those 10 gobs as obvious as it was indeed stood no chances and those flying body parts just filled my underground refuse stockpile (i always worry about a stealth necromancer , i remember a nasty surprise i got in 34.11)
One of my dwarf went in trance mid fight and sliced the same poor gob 3 times , an arm, a leg and the neck that sailed in arc, losing its neck oddly didn't killed him on spot, he died from head getting smashed by another of my warriors.